Field of the Invention
An embodiment of the present invention relates to a testing technology of a turbine blade by an ultrasonic flaw detection test.
Description of the Related Art
An ultrasonic flaw detection test is a technology for nondestructively checking soundness of a surface and an inside of a structural material and is an indispensable testing technology in various fields.
In a phased-array ultrasonic flaw detection test (PAUT), oscillation timing (delay time) of each of a small-sized piezoelectric elements arrayed in plural is shifted, ultrasound waves having an arbitrary waveform is transmitted, and echo waves are received.
The phased-array ultrasonic flaw detection test (PAUT) can handle test targets with complicated shapes and is widely used from the view point of reduction in the number of work processes as compared with a monocular probe that can transmit ultrasound waves only at a predetermined angle.
Turbines such as large-sized power generators and the like by thermal power and nuclear power have a structure in which a base end of a rotor blade is fastened to a large-sized rotor.
Defects such as fatigue cracking or stress corrosion cracking (SCC) caused by long-term operation of the turbine greatly affect safety and early and reliable detection is in demand.
A large-sized blade mounted on a last stage of a large-sized rotor is formed with a base end portion of the large-sized blade having a fork shape, inserted into an outer periphery of a rotor wheel extending having a flange shape from a rotor shaft and is fastened by a pin.
In the turbine blade fastened as above, there is a concern that defects occur in a concentrated manner around a pin hole located on an outermost periphery of the rotor wheel, and reliable test of the position is required.
In a generally performed test having been conducted so far, all the turbine blades are withdrawn from the rotor and surface test technologies such as MT (magnetic particle test) and PT (penetrant test) are applied to each of them in order to ensure test reliability.
However, if such surface test technologies are applied, long time is needed for inspection, and occurrence of new nonconformity caused by disassembling is concerned, which involves not a few demerits.
Thus, a method for testing the turbine blade in a fastened state is proposed (for example, Japanese Patent No. 4694576).
In the prior-art technology in which piezoelectric elements arrayed in plural in a phased array ultrasonic flaw detection sensor are directly brought into contact with a surface portion of a turbine blade still in the fastened state, since most of the surface portion of the turbine blade exposed in the fastened state is curved, a spot with which the sensor can be brought into contact is limited, which is a problem.
Presence of a flat planar portion on the surface of the turbine blade is limited to a tongue portion or the like.
If an incident point of the ultrasound waves is limited to such flat planar portion, a reaching range of the ultrasound waves is largely limited, and the test on the entire region around the above-described pin hole cannot be conducted sufficiently.
Moreover, in the prior-art technology in which a support structure of the plurality of piezoelectric elements transmitting/receiving the ultrasound waves is made flexible and brought into contact with the piezoelectric elements to a curvature portion on the surface of the turbine blade in order to handle the curved shape, a sensor probe having a dedicated structure needs to be produced, and it is difficult to bring such sensor probe into contact with a narrow and small point on the turbine blade, which is also a problem.